1. Field of the Invention
The present invention relates to a grinding machine for use in grinding plate-like objects such as semiconductor wafers.
2. Related Art
Referring to FIG. 7, a plate-like object such as a semiconductor wafer W is attached to a chuck table 60 with its rear side up by using a protective tape T between its front side and the top surface of the chuck table 60. The rear side of the semiconductor wafer W is ground by a grinding means 70.
The grinding means 70 comprises a rotary spindle 71, a mount 72 integrally connected to the rotary spindle 71 and a grinding wheel 73 fixed to the mount 72. The annular grinding wheel 73 has pieces of grindstone 74 fixed to its lower surface, as seen from FIG. 8. While the grinding wheel 73 is made to rotate, the grinding means 70 is lowered until the pieces of grindstone 74 have been applied to the rear side of the semiconductor wafer W under pressure, thereby grinding the rear surface of the semiconductor wafer W.
The semiconductor wafer W is coarse-ground until it has a predetermined thickness, and then the coarse-ground semiconductor wafer W is fine-ground so that it may have a smooth flat surface. The grinding machine is equipped with two grinding means 74, which are provided with pieces of coarse- and fine-grindstone respectively.
Referring to FIG. 9, a turntable 80 has plural chuck tables (three chuck tables 83, 84 and 85 in the drawing) rotatably supported thereon. By turning the turntable 80 about its center of rotation 80a, selected chuck tables are brought to and put below first and second grinding means 81 and 82, which carry out coarse-grinding and fine-grinding, respectively. The chuck tables 83, 84 and 85 can rotate about their pivots 83a, 84a and 85a. 
As seen from FIG. 9, the first grinding means 81 and the second grinding means 82 are so positioned relative to each other that the straight line L1 passing through the center of rotation 81a of the first grinding means 81 and the center of rotation 84a of the chuck table 84, which is put below the first grinding means 81, may be parallel to the straight line L2 passing through the center of rotation 82a of the second grinding means 82 and the center of rotation 85a of the chuck table 85, which is put below the second grinding means 85. The semiconductor wafer W fixedly held by the chuck table 84 is coarse-ground by the first grinding means 81 whereas the semiconductor wafer W fixedly held by the chuck table 85 is fine-ground by the second grinding means 82.
Semiconductor wafers can be put in and taken out from the area at which the chuck table 83 is positioned. Thus, a finished semiconductor wafer can be removed from the chuck table when it is brought to the area, and an unfinished semiconductor wafer can be put on and fixedly attached to the chuck table while it is located there.
Referring to FIG. 9 again, pieces of grindstone 93 set on an annular grinding wheel 92 of the second grinding means 82 pass through the center of rotation 85a of the chuck table 85 to rub against the semiconductor wafer W evenly while the chuck table 85 rotates about its center of rotation. Thus, a semiconductor wafer of predetermined thickness results.
Referring to FIG. 10, the chuck table 83, 84 or 85 has a circular conical surface 83b, 84b or 85b formed on its top. For example, the chuck table is 200 mm in diameter, and the circular conical shape is 10 xcexcm high at its center. Now, it is assumed that the rotary axis 84a of the chuck table 84 is so tilted by turning its adjustment screws 95 and 96 that the grinding plane 94 defined by the pieces of grindstone 93 of the second grinding means 82 may be parallel to the top surface 84b of the chuck table 84 radially at an annular sector area 91 at which a required fine-grinding is effected on the semiconductor wafer W, as seen from FIG. 11.
When the chuck table 84 was positioned below the first grinding means 81 (see FIG. 9), a grinding plane 88 defined by the pieces of grindstone 87 of the first grinding means 81 was not parallel to the top surface 84b of the chuck table 84 radially at an annular sector area 90 at which a required coarse-grinding was effected on the semiconductor wafer W, as seen from FIG. 12.
As a result, the semiconductor wafer W was coarse-ground to be concave more or less, thus making its thickness uneven. Then, the concave wafer is subjected to the fine-grinding when the chuck table 84 is brought to and put below the second grinding means 82. Even though the grinding plane 94 defined by the pieces of grindstone 93 of the second grinding means 82 is kept parallel to the top surface 84b of the chuck table 84 radially at the annular sector area 91, the uneven thickness of the semiconductor wafer cannot be corrected, and therefore, the finished semiconductor wafer of uneven thickness results.
On the contrary, it is assumed that the rotary axis 84a of the chuck table 84 is so tilted that the grinding plane defined by the pieces of grindstone 88 of the first grinding means 81 may be parallel to the top surface 84b of the chuck table 84 radially at the annular sector area 90 at which a required coarse-grinding is effected on the semiconductor wafer W.
When the chuck table 84 is positioned below the second grinding means 82, the grinding plane 94 of the second grinding means 82 is not parallel to the top surface 84b of the chuck table 84 radially at the annular sector area 91 at which a required fine-grinding is effected on the semiconductor wafer W. Accordingly, the precision with which the fine-grinding is effected is lowered. This is the same with the chuck table 83 or 85.
In view of the above, one object of the present invention is to provide a grinding apparatus which is capable of effecting the coarse- and fine-grinding with precision.
To attain this object, a grinding machine comprises: at least a turn table; chuck tables for holding workpieces to be machined, the chuck tables being rotatably fixed to the turntable; a first grinding means for grinding the exposed surface of each work piece held on the chuck table; and a second grinding means for grinding the exposed and first-ground surface of each workpiece. The grinding machine is improved according to the present invention in that the first grinding means includes at least a first grinding wheel having pieces of grindstone so fixedly arranged as to define together a first grinding plane, a first spindle unit having a rotary spindle fixed to the first grinding wheel; the second grinding means includes at least a second grinding wheel having pieces of grindstone so fixedly arranged as to define together a second grinding plane and a second spindle unit having a rotary spindle fixed to the second grinding wheel, and the first and second grinding means are so arranged that the grinding area formed on the workpiece by the first grinding wheel at the time the workpiece is being ground by the first grinding wheel corresponds to the grinding area formed on the workpiece by the second grinding wheel at the time the workpiece is being ground by the second grinding wheel.
The first and second grinding means may be so arranged that a first angle formed between a linear line connecting from a center of rotation of the turntable to a center of rotation of a selected chuck table when the workpiece is being ground by the first grinding means and a linear line connecting from a center of the selected chuck table to a center of rotation of the rotary spindle of the first spindle unit when the workpiece is being ground by the first grinding means is equal to a second angle formed between a linear line connecting from the center of rotation of the turntable to the center of rotation of the selected chuck table when the work piece is being ground by the second grinding means and a linear line connecting from the center of rotation of the selected chuck table to the center of rotation of the rotary spindle of the second spindle unit when the workpiece is being ground by the second grinding means.
The first and second angles may be 180 degrees.
Once the first grinding plane provided by the first grinding means has been put in parallel with the wafer-bearing surface of a selected chuck table radially at the confronting annular sector area, it is assured that the wafer-bearing surface of the selected chuck table is put in parallel with the second grinding plane provided by the second grinding means radially at the confronting annular sector area when the turntable is rotated to put the selected chuck table under the second grinding means. Thus, all finished semiconductor wafers can have the same thickness.
Other objects and advantages of the present invention will be understood from the following description of preferred embodiments of the present invention, which is shown in accompanying drawings.